Modulating carrier wave transmitter



Nov. 10, 1936. w. T. DITCHAM I MODULATING CARRIER WAVE TRANSMITTER FiledOct. 10, 1934 2 Sheets-Sheet 1 NOV. 1936. w. T. DITCHAM MODULATINGCARRIER WAVE TRANSMITTER 2 Sheets-Sheet 2 Filed Opt. 10, 1954 PatentedNov. 10, 1936 UNITED STATES MODULATING CARRIER WAVE TRANSMITTER WilliamTheodore Dit cham, Ghelmsford, England, assignor to Radio Corporation ofAmerica, a corporation of Delaware Application October 10, 1934, SerialNo..'747,673 In Great Britain October 13, 1933 7 Claims.

This invention relates to modulated carrier wave transmitters and moreparticularly to socalled series modulated transmitters, i. e.transmitters of the kind in which modulation is effected by applyingmodulating potentials to the input circuit of a thermionic tube or bankof tubes whose internal impedance is connected in series with theinternal impedance of a thermionic tube or bank of tubes operating atthe carrier frequency to be modulated.

From another aspect the invention may be regarded as relating to thattype of modulated carrier wave transmitter wherein the carrier poweroutput is varied (in addition to being modulated) in dependence uponmodulation level. Transmitters of this kind have the advantage ofimproved efficiency and the present invention has for its object toprovide an improved simplified and highly efficient transmitter of thiskind.

For the sake of brevity in description the tube or bank of tubesoperating at the carrier frequency and at which modulation is effectedwill be referred to simply as the higher frequency tube (although, aspreviously stated, there may be more than one high frequency tube) whilesimilarly the tube or bank of tubes in series with the high frequencytube will be referred to simply as the modulating tube.

According to this invention a modulated carrier wave transmittercomprises a high frequency tube, a modulating tube having its internalimpedance in series with the internal impedance of the high frequencytube, means for applying modulating potentials to the input circuit ofthe modulating tube as in the usual way, and means for rectifyingmodulating potentials and utilizing the rectified direct currentpotentials ob tained as part of the grid bias for the modulating tube,the whole arrangement being such that normally i. e. in the absence ofmodulating potentials, the carrier power will be low but the saidcarrier power will be increased due tothe variation in grid bias of themodulating tube when modulating potentials are applied.

The invention is illustrated in the accompanying drawings which show,diagramatically two forms of construction wherein the said invention isapplied to series modulated transmitters in each of which modulation iseffected at a high frequency amplifier stage.

In Figure 1 a modulator tube or modulating potential amplifier tube hasits anode to cathode impedance connected in series with the anode tocathode impedance of a carrier wave relay tube or amplifying tube, bothimpedances being connected in series with a source of direct cur-- rentpotential. Modulating potentials are applied to the control grid of themodulator tube and rectified modulating potentials are impressed on thedirect current grid biasing circuit of the modulator tube to oppose thebias source therein. In Figure 2, amplifying tubes are interposedbetween the source of modulating potentials and the control grid of themodulator tube and additional amplifying and potential reversing tubesare interposed between the rectifier and the direct current circuit ofthe modulator tube.

Referring to Figure 1 modulating potentials derived from a land line orother source (not shown) are applied, for example through a transformerT1 to the grid of a first amplifier tube V1 whose anode circuit isresistance-capacity coupled by elements B1 C1 to the grid of a secondamplifying tube V2 whose anode circuit is similarly resistance-capacitycoupled by elements R2C2 to the grid of a modulating tube V4. The gridcathode circuit of the modulating tube V4 consists of a resistance RgSconnected at one end to the grid, a resistance Rn shunted by a condenserC4, and a grid bias source E4, the source having its positive terminalconnected to the cathode and the elements Rg3 Rg4 and B4 being inseries. The anode of the tube V1 is connected through a condenser G3, aresistance Rg2 and a bias source B3 all in series, to the common cathodepoint of all the tubes so far described and a preferably adjustabletapping T upon the resistance RgZ is connected to the grid of a furtherthermionic tube V3 whose plate is connected through the primary of atransformer T2 to a source of anode potential (not shown). The secondaryof this transformer is connected at each end to a rectifier D1 or D2(for example metal rectifiers may be used) the further terminals of therectifiers being connected together and to the grid side of theresistance R 4. The other side of this resistance Hg} is connected to acentre tapping point on the secondary of the transformer T2. Thusvoltage from the anode of the tube V1 is applied; at a strength whichcan be adjusted by moving the adjustable tapping T to the grid of thetube V3 and, owing to the action of the rectifying circuit including D1and D2 (which is a full wave rectifier circuit) there will be set upacross the resistance R 4 a direct current potential substantiallyproportional to the level of the modulating potentials. The arrangementis such that the grid bias applied to the modulating tube V4 as a resultof the voltage set up across the resistance Rg4 opposes the negativegrid bias due to the bias source B4. The anode of the modulating tube V4is connected through an inductance L to the electrical centre of thecathode of the high frequency tube V5 and the anode of the said tube isconnected through an inductance L1 to the anode potential source as inthe usual way, so that the modulating tube V4 and the high frequencytube V5 are in series across the source of anode potential. Carrierfrequency potentials are applied to the grid circuit of the highfrequency tube V5 as indicated and the anode circuit of this tube isassociated with the usual tuned circuit 00.

The adjustments are such that in the absence of modulating potentialsonly a comparatively small potential exists across the high frequencytube V5 so that the carrier power and the input power will be low. When,however, signal voltages are applied to the grid of the tube V1 thenegative grid bias upon the tube V4 will be reduced, and the carrierpower produced in the output of the high frequency tube accordinglyincreased.

fne resistance Rg in the grid of the modulatng tube and the condenser C4in shunt therewith constitute a tim control circuit, and the timecontrol circuit should be designed to have a period (time constant)which is long compared to the time period of the lowest modulatingfrequency required to be transmitted.

In the modification shown in Figure 2 modulating potentials are appliedthrough transformer T1 to the grid of a tube V1 whose output isamplified by the tube V2 in cascade therewith, and thence passed to thegrid of a modulating tube V4 whose anode-cathode space is in series withthat of a high frequency tube V5, as in the well known series modulationarrangement. The modulating potentials are also applied through atransformer T'2 to a full wave rectifier circuit including rectifiersD'1 D2. The rectified output is applied across a resistance R 4 shuntedby a capacity 0'4, the combination being in series with a bias batteryH4 in the grid circuit of an amplifier tube V4. The voltage set upacross R'gi by a rectifier circuit opposes the bias due to the sourceB4. The anode of tube V4 is connected through a resistance, as shown, toa preferably adjustable tapping T1 on a resistance which is connected atone end to a source of anode potential (not shown) and at the other tothe cathode point. The anode of tube V4 is also connected through aresistance R"g4 to the cathode point and through a bias source B"4 inseries with a resistance R'gii to the grid of a tube V4. The anode oftube V4 is connected (at) through a resistance, as shown, to the anodesource for the tube V4 (b) through a time control circuit R 4C4 to thecathode of the modulating tube V4 and (0) through a bias battery B4 inseries with a resistance Rg3 to the grid of the tube V4.

In operation rectified modulating potentials from the circuit T'2 D'1D'z reduce the negative bias on the grid of V4 which causes the positivevoltage across this tube to decrease in value thereby increasing theeffective negative bias on the grid of V"4 which in turn results inincreasing the positive voltage across V4 and so reduces the effectivenegative bias on the grid of the modulator tube V4. This reduction ingrid bias on the modulator tube results in an increase of the power atthe high frequency tube V5. The resistance capacity combination Rg4C4 isarranged to have a time constant greater than the time period of thelowest required modulating frequency.

Although in the illustrated embodiments full wave rectifier circuits areused (and preferred) full wave rectification is not an essential featureand half wave rectification may be resorted to and effective operationstill obtained.

What is claimed is:

1. In an improved modulation system, a thermionic relay tube having acontrol grid, an anode and a cathode, a circuit applying carrier wavesto be modulated to the control grid and cathode of said tube, amodulator tube having an anode, a cathode and a control grid, a circuitconnecting the anode-to-cathode impedances of said tubes in series witheach other and with a source of direct current potential, a source ofmodulating potential connected to the control grid of said modulatortube, a source of direct current potential connected with the controlgrid and cathode of said modulator tube for maintaining said controlgrid negative relative to said cathode, whereby the impedance betweenthe anode and cathode of said modulator tube is high in the absence ofmodulating potentials, and means for reducing the impedance of saidmodulator tube in the presence of modulating potentials, comprising aresistance connected in series with said last named direct currentpotential source, and a rectifier having input electrodes coupled tosaid source of modulating potentials and output electrodes connected tosaid resistance.

2. A modulation system as recited in claim 1 in which a thermionicamplifier is interposed between said source of modulating potentials andthe input electrodes of said rectifier.

3. A modulating system as recited in claim 1 in which amplifying meansis interposed between said source of modulating potentials and thecontrol grid of said modulator tube.

4. A modulating system as recited in claim 1 in which said rectifier isof the full Wave type.

5. A modulating system as recited in claim 1 in which amplifying andsignal potential reversing tubes are interposed between the output ofsaid rectifier and said resistance.

6. A modulating system as recited in claim 1 in which amplifying devicesare interposed between said source of modulating potentials and thecontrol grid of said modulator tube, and in which other amplifyingdevices are interposed between said source of modulating potentials andthe input electrodes of said rectifier.

'7. A modulation system as recited in claim 1 in which amplifyingdevices are interposed between said source of modulating potentials andthe control grid of said modulator, and in which additional potentialamplifying and reversing devices are interposed between the output ofsaid rectifier and said resistance.

WILLIAM THEODORE DITCI-IAM.

